A float switch is a device used to detect the level of liquid within a tank. Most float switches are“normally closed,”meaning the two wires coming from the top of the switch complete a circuit when the float is at its low point, resting on its bottom clip.
To complete a circuit, float switches utilize a magnetic reed switch, which consists of two contacts sealed in a glass tube. When a magnet comes close to the two contacts, they become attracted to each other and touch, allowing current to pass through. When the magnet moves away, the contacts demagnetize and separate (breaking the circuit).The float encases a sealed magnet, which moves up and down the length of the stem as a fluid level rises and falls.  As the magnet passes by the contacts in the encased reed switch, they touch and complete a circuit between the two lead wires.

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